This invention relates to an improved method of producing hardened granulated material, obtained by mixing at least water, a binder and fly ash and forming granules by any of the usual granulation techniques, and curing at elevated temperatures.
Many patents and publications describe the production of hardened material on the basis of fly ash and a binder. As a binder calcium hydroxide or a product that will produce calcium hydroxide in an aqueous system may be used (e.g. U.S. Pat. Nos. 4,419,312: 4,394,176: 4,490,178: and PCT/GB No. 83/00248).
An embodiment of this reaction is disclosed in Dutch Patent Application No. 8200193, wherein uncured granules and hot fly ash are mixed in a mixer which is also fed with steam. The hot fly ash comes directly from a coal fired plant. The mixture from the mixer is fed to a curing or hardening silo in which a temperature of just below 100.degree. C. is reached. The partially cured granules and fly ash leaving the silo are separated from each other and the fly ash is recyled for use as starting material for producing new granules.
The partially cured granules are passed into an intermediate storage silo where the partially cured granules are further cured by means of steam.
Another embodiment is disclosed in U.S. Pat. No. 4,490,178. Granules of better compressive strength are obtained by subsequently hardening the granules at temperatures between 85.degree. and 212.degree. F.
In said method the granules are hardened under moist conditions by embedding said granules in a mixture of sand, water and calciumoxide. An advantage of this process is that the heat generated by the slaking of calciumoxide is used in direct contact with the granules for curing or hardening of the granules at elevated temperatures.
A mixture of sand and calcium oxide as embedding material provides very high strength granules but is not always economically practical or required. For example, lower strength granules may satisfy some cement block manufacturing requirements. However, the use of spent embedding material is necessary for good process economics, but lower strength granules do not require the use of sand in the granule mixture, and thus spent sand embedding material is not recycled back for granule mixing. It would be economically advantageous to therefore use an embedding material which could be recycled back into the mixture for forming granules.
Also, although all of the required heat for the hardening of the granules can be solely supplied by steam without embedding material, our experiments have shown that the sole use of steam as a heating source for curing the granules without embedding material has various disadvantages.
(i) Handling and storage of non hardened or insufficiently hardened granules is difficult and causes degradation of the granules.
(ii) The condensation of steam involves formation of water causing the granules to stick together which will hinder the separation of the hardened granules at the end of the hardening process.
(iii) Direct contact of the granules with steam can cause cracks in the granules which will lower their final strength considerably.
Thus the use of embedding material is deemed to be advantageous. However, such embedding material must meet specific criteria for curing.